The present silicon memory devices can be sorted by the storage state of the data. One type of the silicon memory devices is the random access memory (RAM), such as DRAM and SRAM. This type of memory device with data reading/writing speed below several ten nanoseconds is generally called quick-access memory. However, the data saved therein will disappear while the power is shut down. Therefore, the storage state of the RAM is volatile. Another type is the read only memory (ROM), such as electrical erasable programmable read only memory (EEPROM) and flash memory. On the contrary, this type of memory device can save the data permanently but has longer reading/writing speed than the RAM. Because the data do not disappear while the power is shut down, the storage state of the RAM is nonvolatile.
Although the current memory devices all have high density (>64 Mbit), the important properties, non-volatility and quick speed, cannot be performed at the same time. Since the demand for ferroelectric random access memory (FRAM) consisting these two merits becomes urgent.
The FRAM has the same speed in reading/writing data with the conventional RAM, so it belongs to the quick-access memory. When writing data, the ferroelectric film is polarized permanently by a electrical pulse so that the data written in is saved forever even that the power is shut down. Therefore, the FRAM is a non-volatile memory device with quick and non-disappeared properties. Moreover, the design of ROM+RAM can be replaced by one FRAM and then the efficiency will be promoted greatly.
Principle of non-volatile ferroelectric memory device for application is using the polarization-electric field (P-E) hysteresis of the ferroelectric film. Because the ferroelectric film has remnant polarization while no external electric field exists, the data do not disappear when shutting down the power. In the practical application, the ferroelectric film capacitor replaces the storage capacitor of SiO2 to distinct the logic “0” and “1” by different remnant polarization (Pr). When the logic is “0”, adding a positive voltage has a difference between Pr and saturation polarization (Ps). While the logic is “1”, the difference is between −Pr and Ps. The current of logic “1” is larger than that of logic “0” so that it is able to be used for determining the data logic.
In the development of the FRAM, how to combine the manufacture process of ferroelectric and silicon integrated circuit is a key point. Especially when the high density memory is developed, the ferroelectric capacitor formed on the gate of the metal oxide semiconductor field-effect transistor (MOSFET) is imperative, so the research in this aspect is extensively noted. In 1950˜1960, scientists use the natural polarization of the ferroelectric ceramics to control (or change) the electric conduction coefficient on the surface of the semiconductor for producing the ferroelectric field-effect transistor memory. Recently, owing to the excellent characteristics of the ferroelectric film, and requirements of both high density G-bite DRAM and non-volatile memory devices so that the FRAM is noted and studied once again.
In view of foregoing reasons, the present invention provides the ultra-thin aluminum oxide to be the insulator for solving charge-injection problem and the material with bismuth layered perovskite structure to be the ferroelectric film due to its non-fatigue behavior and compatibility to the other high-temperature semiconductor process so that excellent memory characteristics can be reached in the MFIS stacked gate capacitor structure.